Adds negative tests to glance api's

It adds negative tests to get_image and delete_image
api's

get / delete -- non-existent image
get / delete -- image where id is null
get / delete -- deleted image
also moves test_get_image_null_id to negative file.

Change-Id: I239084b273e72dca98cb5813af4d8422d66daa5c
Merge "Adds negative tests to glance api's"